Multi-Stage Falling Particle Receivers

ABSTRACT

The present disclosure is directed to multi-stage falling particle receivers and methods of falling particle heating. As the particles fall through the receiver, the particles are periodically collected and released by flow retarding devices. The periodic catch-and-release of the particles falling through the receiver reduces particle flow dispersion, increases particle opacity and solar absorption, and reduces erosion and damage to surfaces caused by direct particle impingement.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 62/658,340, filed Apr. 16, 2018, entitled “Multi-Stage FallingParticle Receivers,” and is related to U.S. Provisional PatentApplication Ser. No. 62/145,136, “Falling Particle Solar Receivers,”filed on Apr. 9, 2015, and to U.S. patent application Ser. No.15/095,738, “Falling Particle Solar Receivers,” filed on Apr. 11, 2016,the disclosures of which are incorporated by reference herein in theirentireties.

STATEMENT CONCERNING FEDERALLY SPONSORED RESEARCH

The United States Government has rights in this invention pursuant toContract No. DE-AC04-94AL85000 between the United States Department ofEnergy and Sandia Corporation, and pursuant to Contract No. DE-NA0003525between the United State Department of Energy and National Technologyand Engineering Solutions of Sandia, LLC, for the operation of theSandia National Laboratories.

The Australian Government has rights in this invention as partialfunding for this work was provided to the Commonwealth Scientific andIndustrial Research Organisation by the Australian Government.

FIELD OF THE INVENTION

The present disclosure is generally directed to solar energy, and moreparticularly directed to falling particle concentrated solar receivers.

BACKGROUND OF THE INVENTION

Solar power systems offer much promise for clean energy, with few, orzero, carbon emissions. These systems collect incident sunlight andconvert this sunlight into a usable form of power, such as heat orelectricity. Solar energy offers a clean, inexhaustible, sustainablesolution to energy demands and has the potential to supply a verysignificant fraction of U.S. and global electricity consumption. Whilethe U.S. and global solar power potential is known to be immense, solarpower systems have not been economically competitive without governmentsupport, to date. Challenges remain to devise solar technologies thatcan lower installation costs, increase power output, and lower themarginal cost per unit energy produced, for a lower levelized cost ofenergy.

Emerging solar power systems include particle solar receivers that heatparticles for energy conversion, energy storage, thermochemicalprocesses, electricity production, and process heating. However,previously disclosed processes and systems are either not efficient incapturing solar energy to heat particles or require complex structuresor fluidization, which increase costs and parasitic electricityconsumption.

The particle receivers utilize solid particles as the heat transfermedium to absorb the incident concentrated solar energy. One type ofreceiver uses a freely falling particle curtain directly exposed toconcentrated solar energy through an aperture open between the particlesand sunlight to heat the particles. However, there are some inherentdrawbacks with the free-falling particles, such as the decreased volumefraction and opacity of the falling particles caused by gravitationalacceleration that increases downward velocity and dispersion and reducesthe residence time of the particles.

Low particle volume fraction increases the transmittance of the particlecurtain. Additionally, solar energy not absorbed by the particles islost either by being reflected back out the receiver aperture to theoutside surroundings or by being absorbed by the walls and materialsother than the particles. This problem is affected by a combination ofmultiple factors including particle falling height, flow rate, andparticle size. Flow instability of the particle curtain is an additionalissue with the free-falling particle receiver. As the particles fall, astable and dense initial clustered formation is perturbed and transformsinto a wider and unstable particle curtain, creating undesired impact onparticle hydrodynamics and heat transfer. Uneven heating across thefalling particle curtain is one of the main drawbacks of a free-fallingparticle receiver.

The need remains, therefore, for falling particle solar receivers andmethods of falling particle heating to address these and otherlimitations and that efficiently capture solar energy to heat particlesfor energy conversion, storage, and thermal processes.

SUMMARY OF THE INVENTION

The disclosure is directed to multi-stage falling particle receivers andmethods of falling particle heating. As the particles fall through thereceiver, the particles are periodically collected in a trough orfunnel, where they are slowly released again.

According to an embodiment of the disclosure, a solar receiver isdisclosed that includes a housing comprising at least one opening forreceiving concentrated solar irradiance and a front wall and a rearwall, one or more openings in the housing for receiving irradiance, anda flow retarding system comprising one or more flow retention devicesdisposed within the housing for receiving and releasing particles as theparticles fall through the solar receiver.

According to an embodiment of the disclosure, a solar heating method isdisclosed that includes providing particles to a solar receiver andcollecting and releasing particles in one or more flow retarding devicesas the particles fall through the solar receiver and are heated by solarirradiance.

An advantage of the disclosed system and method is that it reduces theparticle velocity and amount of vertical dispersion that a free-fallingparticle curtain experiences due to gravitational acceleration.

Another advantage is that the multistage system increases particle flowstability and reduces the impact of wind and chance of dust formation.

Another advantage of the disclosure is that the collection troughs canbe used to mix the particles to enhance heat transfer and uniformity ofthe particle temperatures as they fall through the receiver.

Another advantage over a large array of continuous discrete obstructionsis that the present disclosure uses less material to slow down theparticles, which is beneficial for cost, repair, and maintenance, andthe collection troughs can be positioned so that they are not within thedirect irradiance.

Another advantage is that our invention is designed to cause fallingparticles to impinge on other particles that have accumulated in thetrough, funnel, or ledge below (rather than impinge on the walls orsurfaces of the trough, funnel, or ledge themselves) when beingcollected and slowed down, which protects surfaces from erosion anddamage. The accumulation of particles in the trough, funnels, or ledgescan be controlled actively (motorized) or passively (gravimetriccounterweights, springs, variable slot apertures), as described in theapplication.

Another advantage, is that in some embodiments, collection troughs canalso be designed to be hidden behind the falling parting curtain toavoid direct exposure to the concentrated solar energy.

Another advantage is that our invention enables variable particle massflow rate while still maintaining the desired high opacity. In previoussystems that use a continuous array of porous structures, if theparticle flow rate is lower than the design point, the mesh or porousstructures will be exposed to direct irradiance since the particle flowwill be insufficient to cover all the obstructions. If the particle flowis greater than the design point, particles will begin to waterfall overthe leading edge of the top row of obstructions, and there will besignificant reduction in the opacity of the waterfalling particles dueto free-fall acceleration and dispersion. Our invention can be designedto handle a wide range of flow rates and still perform the periodiccatch-and-release behavior to reduce downward particle velocities andincrease opacity of the particle curtain.

Other features and advantages of the present invention will be apparentfrom the following more detailed description of the preferredembodiment, taken in conjunction with the accompanying drawings whichillustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The figures depict embodiments of the present invention for purposes ofillustration only and are not necessarily drawn to scale. One skilled inthe art will readily recognize from the following discussion thatalternative embodiments of the structures and methods illustrated hereinmay be employed without departing from the principles of the inventiondescribed herein.

FIG. 1A is a perspective view of a multi-stage falling particle receiveraccording to an embodiment of the disclosure.

FIG. 1B is a front view of the multi-stage falling particle receiver ofFIG. 1A.

FIG. 1C is a side view of the multi-stage falling particle receiver ofFIG. 1A.

FIG. 2A is a perspective view of a trough according to an embodiment ofthe disclosure.

FIG. 2B is a top perspective view of the trough of FIG. 2A.

FIG. 2C is a bottom perspective view of the trough of FIG. 2A.

FIG. 2D is a side view of the trough of FIG. 2A.

FIG. 3A shows another embodiment of a trough particle flow retardingdevice according to an embodiment of the disclosure.

FIG. 3B is a cross-sectional view of the trough particle flow retardingdevice of FIG. 3A.

FIG. 4 is a simplified schematic of an embodiment of a flow retardingsystem disposed within a receiver according to embodiments of thedisclosure.

FIG. 5 is a simplified side profile of a multi-stage falling particlereceiver with V-shaped troughs.

FIG. 6 is a simplified side profile of a multi-stage falling particlereceiver with wall-assisted troughs.

FIG. 7 is a simplified side profile of a multi-stage falling particlereceiver with troughs and wall-attached deflectors.

FIG. 8 is a simplified side profile of a multi-stage falling particlereceiver with wall mounted troughs.

FIG. 9 is a simplified side profile of a multi-stage falling particlereceiver with inclined wall and wall-attached troughs.

DETAILED DESCRIPTION OF THE INVENTION

The present disclosure is directed to multi-stage falling particlereceivers and methods of falling particle heating. As the particles fallthrough a falling particle solar receiver (receiver), the particles areperiodically collected and released from rest (near-zero downwardvelocity) through the receiver by a flow retarding system that includesone or more catch-and-release or flow retarding devices that collect andrelease the particles as they continue to flow through the receiver. Invarious embodiments, the flow retarding system may include one or moreflow retarding devices. In some embodiments the flow retarding systemmay include three or more flow retarding devices. In yet otherembodiments, the flow retarding system may include between 10 and 90flow retarding devices determined by factors including, but not limitedto receiver cavity size, particle flow rate and irradiance.

The periodic collection and release prevents dispersion and accelerationas they fall through the receiver cavity and are heated by concentratedsunlight. For example, periodic collection and release in large-scalesolar receiver systems (receiver size ˜0-20 m) can prevent increasedvertical and horizontal dispersion as the particles accelerate undergravity. In addition, the periodic collection and release reducesvertical particle dispersion that may lead to increased lighttransmittance through the particles, and horizontal dispersion may leadto particle loss through the aperture. Both vertical and horizontaldispersion cause increased heat loss resulting in decreased receiverperformance.

In various embodiment, the flow retarding device may be funnels ortroughs, ledges, grooves, wedges, or other structures or surfaces andcombinations thereof that collect, retard and release the particlesfalling through the receiver under gravity. The flow retention devicesare housed in a solar receiver that includes a housing, one or moreopenings in the housing for receiving irradiance or sunlight. In anembodiment, the flow retention devices are one or more troughs disposedwithin the housing for receiving and discharging particles as theparticles fall through the solar receiver. The one or more openings maybe slots, holes or other passages of variable sizes that allow for therelease of the particles. In another embodiment, the flow retardingdevices may be one or more tilted or inclined ledges or surfaces thatcollect and release the falling particles as the particles overfill thedevice (creating a “waterfall” effect”). In yet another embodiment, theflow retarding device may be any two or more of the above discloseddevices.

The present disclosure is further directed to solar particle heatingmethods that include collecting, retarding and releasing, which can becollectively referred to as “retarding” particles as they fall through asolar receiver. The particles may be retarded one or more times as theyfall through the receiver. The particles are retarded to reduce verticalparticle dispersion that may lead to increased light transmittancethrough the particles, and to reduce horizontal dispersion may lead toparticle loss through the aperture.

In various embodiments, the flow retention devices may have no or one ormore openings. An opening is for this disclosure an opening other thanthe top opening for receiving falling particles. As such, in someembodiments, some flow retention devices include no openings so thatcollected particles “spill out” of the top opening. As such, in someother embodiments, the flow retention devices include side and/or abottom opening for releasing particles. The openings are sized for theflow rate so that the particles are retained within the flow retentiondevices for some amount of time greater than zero before being released.In both embodiments, there is some accumulation of particles for somepredetermined amount of time based on flow rate by the flow retentiondevices. For example, troughs having no openings accumulate particlesuntil the particle level reaches a height in the trough that allowsparticles to spill out of the trough. In another example, particlesaccumulate on ledges to a predetermined amount before the particlesspill off of the ledges. For troughs with openings, the particlesaccumulate in the troughs until the particles are released through theopenings. One or more openings may be passively or actively controlledto adjust the size of the openings, such as for example passivelyadjusted by collected weight of particles or actively adjusted byactuators and/or other mechanical devices by an operator based onfactors including, but not limited to flow rate, particlecharacteristics and irradiance.

In an embodiment, a series of particle collection troughs or other flowretention devices may be placed inside a receiver to collect theparticles at intermittent intervals before the particles can accelerateand disperse too much. In another embodiment, one or more particlecollection troughs may be used. The receiver can be comprised of asingle aperture or multiple apertures to accommodate various sections ofthe particle flow as defined by the troughs. By aiming heliostat beamsthrough multiple apertures, direct irradiance on the troughs (which canbe placed in between the apertures) can be minimized to preventoverheating of the troughs. Some amount of incident light on the troughsmay actually be good to heat the particles, and the troughs can betransparent or porous to allow light to heat the particles directly. Insome embodiments, retarding troughs can be protected from directirradiance by overflowing particles that flow over the edge facingincident concentrated solar energy, in other words, the waterfall ofparticles over the edge block/absorb the sunlight from the troughs.

The collection troughs can be designed to accommodate variable particlemass flow rates. The objective is to decelerate the particles beforethey are released again. If the trough contains just a single apertureat the bottom, the particle mass flow is prescribed by the aperturesize. If the particle flow is less than the capacity of the aperture,then the particles will flow through the open aperture withoutsignificant deceleration. If the particle mass flow is greater than thecapacity through the aperture, then the particles will accumulate andoverfill the trough. Particles that impinge and flow over the moundabove the trough will also not be decelerated to the fullest extent. Inan embodiment, the troughs may include vertical slots that enablevariable particle mass flow rates to be collected and released from anear-zero vertical velocity. In other embodiments, variable particleflow rates can be accommodated by designing the retarding troughs toallow one-side overflowing, creating a waterfall effect.

The troughs may mix the particles to enhance heat transfer and provideuniformity of the particle temperatures as they fall through thereceiver. The collection troughs can be opaque, or they can betransparent or porous to allow direct heating of the particles byincident light. In an embodiment, the collection troughs can bepositioned so that they are not within the direct irradiance. Theaperture size of the collection troughs can be fixed, activelycontrolled, or passively controlled to enable variable mass flow ratesof particles to be released from rest or near rest from each trough. Inother embodiments including overflowing troughs, when overflowing isdesigned to occur on the solar irradiance side, the collection troughscan be efficiently protected from solar irradiance by the overflowingparticles. The troughs can be designed to accommodate variable particlemass flow rates while maintaining a high particle curtain opacity. Incontrast, with continuous discrete obstructions, only a small range ofparticle mass flow rates can be accommodated.

Computer modeling has shown that excessive vertical and horizontaldispersion is mitigated as a result of the flow retention devices, suchas periodic trough collectors. As the particles fall, a high opacity ofthe particle curtain is desired to intercept the incident sunlight. Themultistage system increases the particle opacity.

Other embodiments provide for various arrangements and staggering of theflow retention devices through the receiver. The angle of the troughscan be symmetric or skewed toward one size, and the location of thetroughs can be located anywhere within the receiver or against the backwall to mitigate particle loss and the impacts of wind.

The present disclosure if further directed to methods for heatingparticles falling within a multi-stage falling particle receiver. As theparticles fall through the receiver, the particles are periodicallycollected and released, preventing further dispersion and accelerationas they fall through the cavity of the receiver. This is important,especially in large-scale systems (receiver size ˜10-20 m), to preventincreasing vertical dispersion as the particles accelerate undergravity, which leads to increased light transmittance through theparticles and horizontal dispersion, which may lead to particle lossthrough the aperture.

FIGS. 1A-1C shows different views of a multi-stage falling particlereceiver (receiver) 100 according to an embodiment of the disclosure. Ascan be seen in FIGS. 1A-1C, the receiver 100 includes a housing 102 anda flow retarding system 104 disposed therewithin. The housing 102includes a front panel 103A and a rear panel 103B. The housing 102further includes an inlet 106 disposed within a housing top structure107 and an outlet 108 disposed in a collection bin 109 in fluidcommunication to an internal chamber or cavity 110. The housing 102further includes windows, apertures or openings 112 in the front panel103A that allow directed, concentrated solar irradiance 113 to enter thecavity 110 and heat particles (not shown) falling therethrough. Theconcentrated solar irradiance 113 is from a solar source (not shown),such as a plurality of mirrors. In this exemplary embodiment, the frontpanel 103 includes four openings 112 that allow concentrated solarirradiance 113 to enter the cavity 110, and those openings 112 areseparated by portions of the front panel 103 arranged to block theconcentrated solar irradiance 113 from impinging on the flow retardingdevices 104. FIG. 1B shows a frontal view wherein the flow retardingsystem 104 is visible, however, it should be noted that as seen from theperspective of the angle of irradiance of sunlight into the receiver,the flow retarding system 104 would be not visible as it would beblocked by portions of the front panel 103A In other embodiments, thereceiver 100 may include one or more openings that may or may not bearranged within the front panel to block solar irradiance from impingingon the flow retarding devices.

The flow retarding system 104 retards particles falling through thereceiver 100. In this exemplary embodiment, the flow retarding system104 includes three flow retarding devices 105, respectively referred toas troughs 104A, 104B and 104C. In another embodiment, the receiver 100may include one or more flow retarding devices. In yet anotherembodiment, the receiver 100 may include two or more flow retardingdevices.

In this embodiment, the flow retarding devices 104 are funnels ortroughs 104. The troughs 104 collect and retain the falling of particlesfor a predetermined amount of time, and then release and allow theparticles to continue to fall. In such a manner, the particles fall isretarded. As the particles fall and horizontally (measured from thefront or opening side of the receiver to the opposing back of thereceiver) disperse, the particles are collected by the trough andreleased in a curtain, veil or other shape that has a predeterminedhorizontal length. In such a manner, the falling particle dispersementcan be corrected to a predetermined width. In an embodiment, thepredetermined width is the same as the initial width the particlecurtain has as the particles enter the chamber 110. In anotherembodiment, the predetermined width is less than or greater than theinitial width the particle curtain has as it enters the chamber 110. Insuch a manner, the particle curtain width can be controlled toaccommodate the spatially non-uniform solar flux distribution enteringthe receiver.

The troughs 104 are disposed within the housing 102 and verticallyarranged so that the most upper trough 104A receives particles from theinlet 106, and outputs or releases those particles to a next in sequenceor second trough 104B disposed underneath thereof. The next in sequence,trough 104B thereafter releases those collected particles to a thirdtrough 104C, which releases those particles to the collection bin 109 orother particle collection device or system located proximate the bottomof the receiver 100.

In such a manner, particles falling into the upper most or first trough104A receive irradiance, and the particles falling between the troughsand from the third trough also receive irradiance. In other embodiments,two or more openings 112 may be placed to allow irradiance to bereceived by particles at two or more locations in the falling particlearrangement. A single aperture could also be used with multipleheliostat aim points to heat the particles falling between collectiontroughs.

FIGS. 2A-D show a trough 200 according to an embodiment of thedisclosure. As can be seen in FIGS. 2A-2D, the trough 200 includeapertures, slots or openings 204. In this exemplary embodiment, thetrough 200 includes a top opening 202 for receiving falling particles,and a plurality of apertures, slots or openings 204 for releasingcollected particles. In this exemplary embodiment, the trough 200includes six openings 204. In other embodiments, the trough 200 mayinclude one or more openings 204. In this exemplary embodiment, theopenings are horizonal slots having a length L approximately equal tothe length of the inlet 106.

The size of the slot apertures can be varied to accommodate more massrelease as the slots progress towards the top. The configuration andsize of the slot apertures may be optimized to create particle releaseconfigurations that take advantage of volumetric heating. Furthermore,the collective opening area is sized so that the total volume release isgreater than the mass flow into the receiver. In such a manner, a troughcannot overflow from not being able to release more than the receivedvolume of flow. In addition, the spacing of the slots from the front(side closest to impinging light) to the back (side farthest fromimpinging light) can be varied so as to maintain the width of theparticle flow curtain, increase or decrease the width. Furthermore, thedistance between the troughs can be varied to optimize the heating ofthe particles in conjunction with the irradiance distribution from theheliostat aiming strategy. In particular, the troughs are separatecomponents that do not form a continuous retention of particle fallingflow but provide for a discontinuous catch-and-release system ofretardation.

Referring back to FIG. 1C, it can be seen how particles (indicated bythe mass collected in the troughs and dashed lines) collect and areretained for a period of time greater than zero in the troughs 104 andare released from the openings to continue to fall within receiver 102.

FIGS. 3A and 3B show another trough 300 according to an embodiment ofthe disclosure. FIG. 3B is a cross-sectional view of FIG. 3A and is amirror image of the opposing section. As can be seen in FIGS. 3A and 3B,the trough 300 includes a front panel 310 and a rear panel 312. Thetrough 300 further includes a top opening 314 for receiving fallingparticles, and a bottom opening 316 for discharging or releasingparticles. The trough 300 also includes a central divider 318 thatdirects falling particles either towards the front panel 310 or the rearpanel 312.

The central divider 318 includes a front divider panel 320 and a reardivider panel 322. As can be seen in FIG. 3B, the front divider panel320 includes slots or openings 324 that allow particles directed towardsthe front panel 310 to accumulate and be released into the bottomopening 316 for discharge. The rear divider panel 322 mirrors the frontdivider panel 320. This particular embodiment causes the particles to bereleased through a single opening, creating a single particle curtain.If the particle flow rate exceeds the discharge capacity of the bottomslot of panel 320, the particles will further accumulate and dischargethrough the slot above the bottom slot of panel 320, and so on. Theheight and number of the slot openings in panel 320 can be variable anddesigned to handle any range of expected particle flow rates of thesystem. In one embodiment, there could be as few as one slot opening infront divider panel 320 and rear divider panel 322.

According to another embodiment of the disclosure, the flow retardingsystem may include one or more flow retarding devices that are inclinedplanes. FIG. 4 is a simplified schematic of an embodiment of a flowretarding system 400 disposed within a receiver 402. The flow retardingsystem includes flow retention devices 404 that are planes or ledgesthat are. In this exemplary embodiment, the ledges are inclined downwardfrom the rear to the front of the receiver 402. In other embodiments,the ledges may be horizontal or inclined downward. In such a manner,falling particles 406 can cascade through the interior of the receiverwhile being irradiated by concentrated solar irradiance. In thisexemplary embodiment, the inclined planes are inclined downward byfifteen degrees. In another embodiment, the inclined planes may beinclined between 0 and less than 90 degrees. The inclined planesminimize unnecessary particle collection.

FIG. 5 illustrates a simplified schematic of a flow retarding system 500arranged within a solar receiver 501 having a rear wall 510 according toanother embodiment of the disclosure. As can be seen in FIG. 5, the flowretarding system 500 includes troughs 502 having a V-shaped profileformed by opposing walls 504 with an aperture or opening 506 at thebottom of the troughs 502. The troughs 502 are installed in series fullyexposed to solar irradiance 508. In this exemplary embodiment, thetroughs are vertically offset. In other embodiments, the troughs may bevertically alighted. In order to maintain proper particle level to allowefficient retardation and stable release of particles, the troughopening 506 can be controlled actively by an actuator or passively byparticle weight in the trough. For example, the troughs may be activelyor passively controlled to allow the trough sides to swing as shown bythe arrows to increase or decrease the bottom opening size in order tocontrol flow rate and the amount of accumulation of particles 550(indicated by the mass shown contained within the troughs.

FIG. 6 illustrates another simplified schematic of a flow retardingsystem 600 arranged within a solar receiver 601 having a rear wall 610according to another embodiment of the disclosure. As can be seen inFIG. 6, the flow retarding system 600 includes ledges 602 inclined orangled downward towards the rear wall 610. The reference system in thisand other figures disclosed herein, referring to FIG. 1, is that thedirection from the inlet 106 to the outlet 108 is downward. The inclinedledges 602 and rear wall 610 define an opening 606 that allow particlescollected on the ledges 602 to flow towards the rear wall 610 and thendownward. The inclined ledges 602 may be actively or passivelycontrolled to adjust incline angle and/or distance from the rear wall610 to control particle flow. The inclined ledges 602 include aninclined flat portion 602 a and a barrier portion 602 b. The barrierportion 602 b includes ridges, walls, or other flow impediments thatslow or impede the flow of particles off of the ledges 602. In otherembodiments, the ledges 602 may only include an inclined flat portionand no barrier portion. In other embodiments, the inclined ledges 602may only include a barrier portion and no flat portion. As shown in FIG.6, the angle of the troughs can be actively or passively increased ordecreased as shown by the arrows to control the amount of particlesaccumulated 650 on the ledges and to control the release or flow ratefrom the ledges.

FIG. 7 illustrates another simplified schematic of a flow retardingsystem 700 arranged within a solar receiver 701 having a rear wall 710according to another embodiment of the disclosure. As can be seen inFIG. 7, the flow retarding system 700 includes flow diversion structures709 and corresponding flow retarding devices 702. The flow diversionstructures 709 include an inclined surface 709 a that divert fallingparticles downward away from the rear wall 710 and into the flowretarding devices 702. The flow retarding devices 702 have anon-symmetrical V-shaped cross-section including a high side 702 a and alower side 702 b that directs particles filling the flow retardingdevices 702 towards the rear wall 710. The flow retarding system 700allows efficient retardation and stable release of particles regardlessof particle flow rate without needing an active or passive flow controlmechanism in the trough. Direct particle falling through the gap betweenwall and trough is prevented by deflectors attached to the wall justabove each trough. It should be understood that particles fill the spaceabove and contained by the flow retarding devices 702 in a manner thatwhen the space if full, the particles impact contained particles, andmay mix, and then particles overflow as shown by the flow represented bythe dashed line.

FIG. 8 illustrates another simplified schematic of a flow retardingsystem 800 arranged within a solar receiver 801 having a rear wall 810according to another embodiment of the disclosure. As can be seen inFIG. 8, the flow retarding system 800 includes flow retarding devices802 configured to release particles in a cascade flow away from the rearwall 810. The flow retarding devices 802 include a trough portion 804and a mount portion 808 that connects the trough portion 804 to the rearwall 810. The trough portion 804 increases in length as the flowretarding devices 802 progress downward, allowing for particles tocascade. In this exemplary embodiment, the first (numbering from top tobottom) flow retarding device has not mount portion. In otherembodiments, the mount portions may start with zero or greater than zerolength. In such a manner the flow retarding devices are designed toallow overflowing of particles in the direction of solar energyreception. In this exemplary embodiment, the mount portions are inclineddownward in order to minimize unnecessary particle collection. Particlesmay impact and accumulate over the mount portion 808 and/or troughportion 804. In other embodiments, the mount portions may be horizontalor inclined downward. The flow retarding system provides for efficientretardation and stable release of particles regardless of particle flowrate. An additional benefit of this design is that the troughs andmounts are protected from direct solar irradiance by overflowingparticles. The protected trough prevents heat loss from the particlesand material damage caused by direct exposure of trough to high fluxsolar irradiance. It should be understood that particles fill the spaceabove and contained by the flow retarding devices 802 in a manner thatwhen the space if full, the particles impact contained particles, andmay mix, and then particles overflow as shown by the flow represented bythe dashed line.

FIG. 9 illustrates another simplified schematic of a flow retardingsystem 900 arranged within a solar receiver 901 having a rear wall 910according to another embodiment of the disclosure. As can be seen inFIG. 9, the flow retarding system 900 includes flow retarding devices902 configured to release particles in a cascade flow away from the rearwall 910. The flow retarding devices 902 include a retention portion 904and a mount or base portion 908 that connects the retention portion 904to the rear wall 910. In this exemplary embodiment, the rear wall 910 isinclined downward towards the direction of incident light, allowing forparticles to cascade. In such a manner the flow retarding devices inconjunction with the inclined rear wall are designed to allowoverflowing of particles in the direction of solar energy reception thatprotect the devices from direct solar irradiance. Optimum angle of therear wall inclination is determined by a combination of particle flowrate and the distance between troughs. It should be understood thatparticles fill the space above and contained by the flow retardingdevices 902 in a manner that when the space if full, the particlesimpact contained particles, and may mix, and then particles overflow asshown by the flow represented by the dashed line.

The disclosed embodiments include multiple flow retarding devices,however, it should be understood that a flow retarding system mayinclude one or more of one or more of the various embodiments of flowretarding devices.

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

What is claimed is:
 1. A solar receiver, comprising: a housing comprising at least one opening for receiving concentrated solar irradiance and a front wall and a rear wall; one or more openings in the housing for receiving irradiance; and a flow retarding system comprising one or more flow retention devices disposed within the housing for receiving, accumulating and releasing particles as the particles fall through the solar receiver.
 2. The solar receiver of clam 1, wherein the one or more flow retention devices are selected from a group consisting of troughs and ledges.
 3. The solar receiver of claim 1, wherein the flow retarding system further comprises a flow directing device inclined attached to the rear wall.
 4. The solar receiver of claim 2, wherein the one or more flow retention devices are troughs and the troughs are positioned within the housing so that the troughs are not irradiated by concentrated solar irradiance entering the receiver.
 5. The solar receiver of claim 2, where the one or more flow retention devices are troughs and the troughs comprise one or more openings for releasing the particles.
 6. The solar receiver of claim 5, wherein the one or more openings release particles to a particle outlet at the bottom of the trough.
 7. The solar receiver of claim 2, wherein the flow retention devices are troughs and one or more of the troughs are connected to the rear wall by mounts inclined downward away from the rear wall.
 8. The solar receiver of claim 2, wherein the flow retention devices are troughs configured to discharge particles away from the rear wall creating a cascade of particles away from the rear wall.
 9. The solar receiver of claim 2, wherein the one or more flow retention devices are ledges.
 10. The solar receiver of claim 9, wherein the ledges are inclined downward towards irradiance entering the solar receiver creating a cascade of falling particles.
 11. The solar receiver of claim 9, wherein the ledges are horizontal.
 12. The solar receiver of claim 9, wherein the ledges comprise an inclined portion and a flow barrier portion.
 13. The solar receiver of claim 9, where the ledges are configured to direct particle flow downward towards the rear wall.
 14. The solar receiver of claim 1, wherein the rear wall is inclined downward towards the direction of solar irradiance.
 15. The solar receiver of claim 1, wherein one or more of the one or more flow retention devices are actively or passively controlled to adjust particle flow.
 16. A falling particle solar heating method, comprising providing particles to a solar receiver; and collecting, accumulating and releasing particles in one or more flow retarding devices as the particles fall through the solar receiver and are heated by solar irradiance.
 17. The method of claim 16, wherein the one or more flow retarding devices are toughs.
 18. The method of claim 16, wherein the one or more flow retarding devices are ledges.
 19. The method of claim 16, wherein the one or more flow retarding devices create a cascade of falling particles.
 20. The method of claim 19, wherein the cascade of falling particles protect the one or more flow retarding devices from concentrated solar irradiance entering the solar receiver.
 21. The method of claim 17, wherein the troughs include openings for discharging collected particles to a bottom outlet. 